Metastable austenitic stainless steel

ABSTRACT

A metastable austenitic stainless steel containing 0.07 to 0.18 percent carbon, 0.9 to 6.2 percent manganese, 4.1 to 7.7 percent nickel, 14.1 to 17.9 percent chromium and 0.01 to 0.14 percent nitrogen, the balance essentially iron. The steel has an Instability Function (IF) of 0.0 to 2.9 as determined by the following equation: IF + 37.193 - 51.248 (% C) - 1.0174 (% Mn) - 2,5884 (% Ni) - 0.46770 (% Cr) - 34.396 (% N).

Waited States Patent 72 I inventors Kenneth G. Bricimer Wilkinsburg Borough; David C Ludwigson, Hempiieid Township, Westmoreiimd County, both 0!, Pa. [21 1 Appl. No. 693.205 [22 1 Filed Dec. 26, 1967 {45] Patented Aug. 17, 1971 {73] Assignee United States Steel Corporation (54] METASTABLE AUSTENTTIC STAINLESS STEEL 4 Claims, 1 Drawing Fig.

[51 1 Int. Cl 1 ..C22c 39/20, 823p 17/00 [50] FieldloiSearch...... 75/128 A, 128, l28.5;29/527.7

{56] References Cited UNITED STATES PATENTS 2,820,708 [/1958 Waxweiler 75/128 3,152,934 10/1964 Lula 75/128 A 3,259,528 7/1966 Carlsen 75/128 3,253,966 5/1966 Malagari IS/128.5 3,276,864 10/1966 Loginow 75/1285 Primary Examiner-Hyland Bizot Attorney-John R. Pegan IF +37.l93

-51.248 C) l.0174(% Mn) 2 .5884 Ni) 0.46770 Cr) -34.396 N).

car 0 ROLLED re 00 ns/ Q YIELD smnvsm cow ROLLED r0 I20 x51 20? 1151.0 srweracru ANIl/EALED STABL E STEELS ANNEALED STEELS SLIGHTLY MDDERATELY l METASTABLE METASTABLE '1 'l STABLE INSTABILITY FUNCTION ELONGAT/O/V IN 2 INCHES, PERCENT PATENT EU AUG I 7 I97! ANNEALED STEELS COLD ROLLED r0 00 k8! YIELD STRENGTH COLD ROLLED T0 /20 KS/ new sr/mvam 20- v 001.0 ROLLED ro/so xs/ YIELD STRENGTH 1o- SLIGHTLY MODE/M TELY b swung JM7W$TABLE L META$TABLE 1 I x A I l J l l 5 -4 3 ,-2 0 +1 +2 +3 +4 +5 +6 INSTABILITY FUNCTION INVENTORS.

KENNETH 6. BRICKIVER and DAV/0 C. LUDW/GSO/V Attorney METASTABLE AUSTENITIC STAINLESS STEEL This invention relates to a metastable austenitic stainless steel sheet and strip with improved forming characteristics.

Metastable austenitic stainless steels are those stainless steels that have an austenitic structure (face-centered arrangement of atoms) in the annealed condition, but are capable of undergoing a progressive solid-state transformation to a martensitic structure (body-centered arrangement of atoms) during mechanical deformation. The good ductility usually exhibited by such steel accounts for their widespread use in the production of formed parts. A typical metastable austenitic stainless steel is AlSl Type 301.

Manufacturers of formed parts from metastable austenitic stainless steels desire that the steel possess great ductility but limited ultimate strength. Great ductility permits the steel to be formed into the intricate shapes aesthetically pleasing objects that so often require severe forming strains. Limited ultimate strength assures w forming stresses, or minimum resistance to fonning, low wear on forming dies and equipment, low expenditure of energy during forming, and minimum springback in severely formed articles. Great ductility and limited ultimate strength are of particular importance in annealed sheet and strip, from which a majority of parts are formed. However, these qualities are also important in strip cold rolled to a high-yield strength level. Such strip is used when the strength level desired in the finished part must exceed some minimum value. But cold rolling, to achieve a high yield strength, reduces ductility and increases ultimate strength. In cold-rolled strip, manufacturers desire the greatest ductility and the lowest ultimate strength available at a given yield strength level.

The present invention provides a metastable austenitic stainless steel sheet and strip that exhibits superior combinations of high ductility and low ultimate strength. Moreover, these combinations of properties, for both annealed sheet and strip and cold-rolled sheet and strip, exist in a steel of the same composition so that fewer steels may be useful for different end applications.

The steel of the present invention is essentially an iron-base alloy that contains carbon, manganese, nickel, chromium and nitrogen as major alloying elements. It has been discovered that excellent formability is achieved in such a steel when two conditions are met: (1) the steel has an instability function (a measure of a steels propensity to transform from austenite to martensite upon straining) within a restricted range; (2) the concentration of each major alloying element is within a specified range.

The instability function (1F) is defined by the following equation:

-5 1.248 C) -1.0174(% Mn) -2.5884 Ni) 0.46770 Cr) 34.396 N) The principal feature, and absolute requirement, of the invention is a composition such that the IF has a value between zero and 2.9.

Steels that exhibit lF values between zero and 2.9 are defined as slightly metastable" austenitic stainless steels. Such steels exhibit levels of ductility appreciably better than those of wholly stable austenitic stainless steels, i.e. those that exhibit negative 1F values. Also, such steels exhibit levels of ductility appreciably better than those of "moderately metastable austenitic stainless steels which are defined as those that exhibit 1F values greater than 2.9.

Among stainless steels so constituted that the IF is within the aforementioned critical limits, are those having 0.07 to 0.18 percent carbon. Among such steels, the ranges for the other major alloying elements are rather wide: manganese, 0.9 to 6.2 percent nickel, 4.1 to 7.7 percent; chromium, 14.1 to 17.9 percent; and nitrogen, 0.01 to 0.14 percent. The IF is applicable within these broad ranges of composition and the excellent formability observed in steels with IF values inside the critical limits is available within these broad ranges of composition.

The critical limits specified for the IF are highly restrictive but include groups of steels with identical manganese, nickel, chromium and nitrogen contents. IN this group of steels, the maximum carbon content range permitted by the limits on the [F is a part of the overall range within the invention for this element, i.e. 0.07 to 0.18 percent. To utilize a wider portion of the overall carbon range and still achieve the excellent formability available within the limits set on the IF, compensatory adjustments in one or more of the other major alloying elements will keep the IF within the critical limits. When such adjustments are made, however, excellent formability is obtained within the overall range for carbon. Likewise, when all major alloying elements are considered, only a very small fraction of all possible combinations of these elements within their overall ranges satisfy the limits imposedby the IF (equation). Nevertheless, the excellent formability achieved when the 1F is between zero and 2.9 is available at least within the overall ranges specified. 7

Among steels so constituted that their 1F is between zero and 2.9, excellent formability is achieved within the overall ranges described above. However, optimum formability is approached among such steels as the overall range of one or more of the major alloying elements is in the following preferred range limits as follows:

0.10 to 0.12 percent C 0.90 to 1.10 percent Mn 6.75 to 7.25 percent Ni 16.6 to 17.8 percent Cr 0.07 to 0.09 percent N The invention will be more fully described by the following examples:

Three series of austenitic stainless steels were prepared having the compositions given in tables l-A, B and C.

TABLE I-A Compositions of the First Series 01 Stools Chemical analysis, percent Heat No. C Mn P S Si Ni Cr N P8206-1 0. 063 1. 69 0. 019 0.015 0. 53 6. 16. 2 0.033 0. 11 0.93 0. 020 0. 018 0. 51 6. 70 16. 3 0. 029 0. 064 O. 92 0. 018 0. 015 0. 51 6. 67 16.2 0. 081 0.12 1.62 0.021 0.015 0.41 6. 67 16.1 0.084 0. 066 0. 94 0. 020 0. 014 0. 49 6. 79 17. 3 0.036 O. 13 1.64 0.017 0.018 0.53 6. 67 17.1 0.033 0. 066 1. 62 0.020 0. 014 0.45 6. 82 17.3 0.078 0. 13 0. 94 0. 020 0. 018 0. 52 6. 65 17. 3 0.076 0. 067 0.94 0. 019 0. 018 0. 50 7. 48 16. 0 0.029 0. 12 1. 62 0. 022 0. 015 0. 52 7. 72 16. 2 0. 032 0. 072 1. 64 O. 021 0.018 O. 50 7. 58 16.2 0.071 0. 11 0.97 0.019 0.016 0.44 7. 64 16.2 0.069 0. 043 1. 61 0.017 0.015 0.53 7. 60 17.5 0.034 0. 11 0.93 0. 021 0.016 0. 51 7. 70 17.3 0.040 0. 068 0.93 0. 024 0. 016 0. 51 7. 55 17. 0 0. 059 0. 12 1. 65 0.019 0. 017 0. 52 7. 65 17. 2 0.078 0. 039 1.30 0.019 0.015 0.53 7. 17 16.5 0. 050 0.15 1.31 0.018 0.015 0. 49 7.13 16.8 0.056 0.10 1.35 0.017 0.017 0.51 7.00 16.7 0.019 0. 10 1. 31 0. 021 0.015 0. 52 7.08 16.6 0.10 0. 0111 1. 30 0. 0111 11. 014 0. 46 7.17 15. 6 0. 060 0. l0 1. 28 (1.0111 0. (118 0. 51 7.14 17.7 0.052 (1. 111 1. 26 0. 0111 11. 911i 0. 50 6.10 16. 5 0. 053 0. 0112 1. 26 11. 11111 1). (115 (1. 53 8. 15 16.8 t). 052 (1. 075 ll. 51 11. (118 0. 015 (1. 54 7. 22 16. .1 0. 058 l). (190 J. (111 11. (121 (1. (114 (1. 47 7. 24 16. 7 (l. 054 (1. 0215 1. .26 (1. 017 (1. 015 0. 50 7.07 16. (1 0. 054 0. 11 1. 311 11. 0111 (1.016 0. 53 7. 14 16. 8 0. 051 0.098 1. 28 0. 019 0.015 0. 51 7. 04 16.4 0. 050 0. 10 1. 27 0.019 0.015 0. 51 7. 10 16.5 0. 046

TABLE I-B Compositions of the Second Serles of Steels Chemical analysis, percent Heat No. C Mn P S Si Ni Cr N P8063-1A. 0.140 6.17 0. 016 0. 018 0. 56 5. 04 14.1 0. 086 T55511 0. 180 6. 03 0. 020 0.009 0. 48 5. 04 14. 4 0.011 T5553-1 0. 5. 94 0. 022 0. 009 0. 51 5. 04 14. 5 0. 054 R9593-2. 0. 5. 98 0.021 0. 018 0. 46 4. 08 17. 11 0. 140 P805843 0. 130 1. 22 0. 017 0. 021 0. 49 7. 03 16. 8 0. 020 R9588-1 0. 058 6. 04 0. (120 (1. 020 11. 47 (i. 111 111. 3 (1. (H3 P8061411"... 0.140 6.10 (1. 016 11.511 5.02 14. 1 0.017

TABLE I-O TABLE IIBContinued Compositions of the Third Series of Steels yield Chemical analysis, percent G ld 2353: Tensile iif reduction, ofiset), strength, inches, Heat c P S1 or N Heat Number percent K 5.1 K s.i percent 3g? {32% 22 T5322-1 2. 63 0.021 0.010 0. 66 9. 01 16.2 0.036 4 8 7 0 T5328-1 1. 29 0. 019 0.010 0. 70 9.05 20.0 0.043 0 6 2079 T5330-1 2. 80 0. 021 0.011 0. 36 9.10 19.9 0. 037 6 206's 247'0 0 $53414 1.81 0. 019 0.012 0. 37 11 1.3 13.1) 0. 8:18

, P8204-1 12. 0 85. 7 180. 3 16. 0 T53-10-1 0. 090 1. 81 0.018 0.013 0. 49 10.9 17. 9 0.039 10 mg 1333 1913 1L 5 32. 9 169. 7 200. 9 10. 0 43. 0 3 8 51.0 .9 .9 Each steel described in table I was melted in a IOO-pound induction furnace, cast into slab-type ingot molds, hot rolled Pam-1 3:; 32; 332% 33:8 and cold rolled to 0.035-inch-thick strip, and annealed for 60 I5 seconds in molten salt at 2,000 F. in addition, annealed strip 9 1 :1; from the first series of heats wasreduced 10, 20. 30. 40 and 50 PBzOH 10.1 83' 9 160.0 36 0 percent in thickness by cold rolling. Product equivalent to that 19. 4 115. 7 175. 7 28. 0 produced in the laboratory can be produced commercially by 28:: i523 conventional stainless steelmaking practices. 20 50.8 211.7 226. 7 8. 5 Sheet specimens from each steel were tested in tension and 1181324 1L 6 85,9 168 5 0 the results are shown in tables Il-A, B, C and D. The elongag-g gg Egg-3 g2 tion values so obtained are used to measure forming ductility. 38.9 182.4 240.8 17.0 The tensile strength values obtained are used as a measure of 4 Wjfliq. 2945.19 f. forming resistance. Yield strength values are used as a mea- P319544 g-g g-g sure of initial resistance to deformation. 5 193:1 5 40. 9 188. 8 198. 6 13. 5 50.8 214. 4 221. 2 9. 0 TABLE II-A P8207-1 12. 1 89. 6 151. 7 39. 0 Results of Tension Tests on Annealed Strip From 30 19.9 119.6 170.4 31.0 the First Series of Steels 30.8 144. 2 187.6 19. 5 40. 3 165. 8 204. 2 15. 0 Yield 49. 1 185. 5 212. 0 17. 0 strength Elonga (0.2 Tensile tion in 2 P8189-1 10.0 82.7 148. 4 40.0 ofiset), strength, inches, 19. 5 117. 3 169. 7 28. 0 K 6.1. K 5.1. percent 30.0 136. 0 179. 4 24. 5 38.1 149.5 203. 7 14.0 32. 9 144. 8 34. 5 51.1 180. 6 213. 5 14. 0 37.8 159. 5 41.9 40. 7 152. 0 34. 2 P812004 11.3 89. 9 146. 1 40.0 46. 8 138. 9 65. 9 21.3 127. 0 176. 9 27. 0 32. 8 147. 4 31. 7 32. 4 158. 7 206. 1 17. 5 41.3 149. 0 55.1 43. 3 173. 4 211. 7 17. 0 42. 9 135. 0 54. 5 50.2 198. 8 228. 5 13.5 49. 4 144.0 61. 8 33. 1 139. 3 38. 8 P8196-2 11.0 73. 2 144. 9 26. 0 39. 1 118. 0 09. 5 19. 2 111. 2 155. 0 20. 0 41. 6 123. 6 63. 6 30.9 145. 8 174. 5 12.0 44. 7 121. 9 69. 7 40.2 164. 7 177. 6 13.0 33. 3 122. 2 46. 5 50. 3 180. 3 186. 7 13. 0 38. 9 123. 8 68. 3 40. 4 130. 7 53. 9 P8202-1 9. 6 75. 7 138. 8 46. 0 47. 4 106. 6 65.0 19.6 115.1 166.1 27.0 34. 4 13131 28. 3 29. 9 148. 0 182. 5 22.0 46. 3 124. 8 71. 6 41.2 175. 9 206. 5 14.0 35. 0 145. 6 49. 5 50. 9 189. 8 211. 5 16. 5 49. 1 122. 3 72. 1 40. 3 144. 3 50.2 P8185-1 10. 3 79. 5 151. 9 35. 5 43. 4 133.9 61. 4 17.5 117. 9 175. 9 24. 0 42. 4 163. 6 40.0 34.1 151. 7 18.56 18.0 41. 6 109. 5 71. 4 40. 0 150. 9 195. 5 16. 5 30. 6 145. 8 42. 4 50. 8 203. 8 222. 8 10. 0 40. 5 127.1 64.1 39. 8 141. 2 52. 1 P8181-1 s. 7 83.8 128. 5 44.0 41.7 138. 2 58. 9 21.1 113.1 167. 7 22.5 40.6 141. 8 55. 7 5 5 29. 8 147. 4 191. 2 18.0 41. 7 140. 3 55. 5 37. 9 162. 7 210. 8 16.0 49. 8 191.1 228. 4 12.0

P8193-1 10.2 56. 7 156. 0 17. 0 TABLE II-B 19.5 104.3 169.1 12.0 30.8 159. 0 184.1 11.0 Results of Tension Tests on Cold-Rolled Strip From the First Series 40.4 182.8 194.2 0.0 51 Ste 5 50.5 221.5 224. 2 2.0

Yield P8190-1 9. x 90. 1 148. 4 42. 5 st rcngtli Elongation 20. 2 1'2]. 1'1 170. 3 30. 0 (old (0.2% 'lonsila in .2 30. 0 1'10. 7 I87. 5 22. 0 reduction. ollsct), strength, inches, l0. 5 175.2 210. 2 l0. 0 H01". Ninnlwr percent K s. K 14.1 percent -19. 8 211.3 247. 7 16. 0 18300-1 12.1 80.3 170. 2 17. 0 P8194-1 11.2 90. 0 174.4 31. 0 23.0 144. 3 193. 9 12. 0 19.2 121.4 192. 8 23.0 31. 2 175. 8 194. 5 12.0 29. 8 149.2 207. 4 18.0 40. 1 190. 0 200. 0 10. 0 40. 1 192. 7 216. 1 16.0 51.2 217. 7 221.2 7. 5 50.8 220. 5 232. 3 10.5 PSlStr-l 10. s 86.2 193. 8 24. 5 P8197-1 10.4 82.8 155. 0 5o. 0 20.8 139.0 220. 2 20. 0 20.6 124. 4 182. 9 30. 0 29. 9 185. 0 231. 9 13. 0 30. 1 147. 5 199. 2 23. 5 39. 8 204. 0 236. 2 12. 0 36. 9 158. 4 196. 3 22.5 50.8 253. 4 264. 6 5. 5 49. 8 191. 5 234. 5 13. 5

P8209-1 9. 4 69. 2 175. 9 23. 5 1'8201-1 1 I. 6 82. o 175, 5 21;. o 19.4 119. 0 196.1 15. 5 20. 3 129. 0 191. 2 21. 9 29.8 156.6 203.7 12.5 32. 9 158.6 219. 3 10.5 40.2 177.1 210.3 13.0 39. 8 I80. 5 221.2 12.5 49. 4 212. 8 230. 2 9. 0 50.2 189. 8 223. 3 13.5

TABLE II-B--Cntlnuc(l timate stress (the average drawing load divided by the average Yield cross-sectional area of the cup sidewall supporting this load) is strength Elongation also given in table lll. Gold (0.2% Tensile in 2 rmiuntinn, offset), strength, inches, Hum. Nlunhur pin-went. K 14.1. K 11.1. percent 1 9194 I 19. 4 11:1. 0 17s. 0 27. 0 TABLE I11 20. 11 107.11 107. 2 2:1. 2 40.4 188 1 221.0 12. 40. 7 231 6 243. 7 12. 0 Results of Deep-Drawmg Tests on the 2. 2 P8192 3 3?} 8 8 Flrst Senes of Steels Ultimate P8210-1 10. 5 80. 0 132. 7 40. 0

20. s 111. 2 161.9 26. 0 cup 28. 9 142. 4 170. 3 22. 0 K 40. 4 166. 8 197. 5 15. 5 50. 0 180. 0 205. 3 11. 5

P8206-l 0.709 140.0 P8208-2 10.6 72.8 168.1 24.0 P8l86-l 0.720 147.9 g'g gi-g 8 P8209-l 0.713 145.1 39: 4 176: 6 207: 9 15: 0 117-1 48. 7 223. 7 234. 3 11.5 P8204-l 0.735 141.3 P8l83-l 0.259. 132.9 P8205-1 11. 5 81.9 149. 7 44.0 mow (L870 13:1 13:3 158:2 i318 9-991 40. 6 177.2 217.1 13. 0 I319 51.7 213.0 236.8 8.5 P8207-l 0.936 112.3 P8195-2 10 1 I 74 7 104 9 32 0 W894 19: 7 126: 4 19119 221 0 2 'g g 1;;

a1. 2 150.1 199. 2 17.0 P 1 39. 7 181.1 214. 2 13. 5 3O P8202-2 0.977 118.0 50. 0 185.0 216. 0 16. 0 92125-1 0.869 121.11 P 1 .89 o .4 12191-1 7 79. o 162. 3 2. 0 23 1 301 2 19813 i 2218 0931 I105 40. 0 165. 0 200. 3 17. 0 P8I94-I 0.831 133.3 50. 4 216. 4 239. 2 8.0 1 1974 0,929 [2 1.0 99201-1 0.710 138.0 P8188-1 9. 3 78. 6 163. 8 39. 0

20. 4 131.1 197. s 20. 0 30,1 153 3 2024 m 0 P8192-l 0.736 155.0 35. 2 167. 0 211. 6 1s. 0 P82 10 0.924 108.1 50. 2 225. 4 244.1 1]. 0 93203-2 0,809 l4l.2 Y 99205-1 0.910 119.0 Pam-1 5&3 3,213 523 2:8 40 P8l95-2 0.1170 133.0 29. 9 160. 4 214. 6 18. 5 P3 l 07892 I300 39. 8 194; 6 229. 5 11.0 P8 1 88-! 0.873 l35.0 50. 8 221. 4 239. 9 10. 5 p 7 I 0 [3L9 T113211 1 6 Results of Tension Tests on Annealed Strip from the Second Series of Steels By employing a magnetlc method, a measure of the propenm sity of each experimental steel in the first series of steels to unstl'tzlgltyl l T l Elonga dergo the strain-induced transformation is obtained. Values of am 0 OD 111 v offset): Strength inches thls measure of 1nstab1l1ty, lF, are shown 1n table IV. Heat No. K 5.1. K s.i. percent V 47. 1 138. 0 67. 5 41. 7 137. 0 05. 0 TABLE IV 45. 2 131.0 61.8 63. 4 123.2 65.0 40. 4 151.3 4 5g. 2 40.0 117. 5 .5 M su e n 37. 4 153- 9 5L2 ea r d l stablllty F unctlons for the First Series of Steels TABLE II-D Results of Tension Tests on Annealed Strip from the Third Series of 1 steels Heat No. Instability Function t Yielld1 El s rengt onga- (0.2% Tensile tion in 2 2325:: :3; ofiset), strength, inches, Heat No. I K s.i. K s.i. percent P8109-l 5.620 P8 199-1 1.77s T5320-1 39. 0 96. 0 55. 5 P320444 5.522 22321-1 s3. 4 87.8 54. 5 PM 3 235 T5322-1 39. 9 97. 1 55. 8 T5328-1 44.0 98.4 54. s 3-519 T5330-1 41.6 94. 9 54. 5 P8 182-] 1.562 T5341-1 33.3 28. i 22.2 P8 19s 1 4.652

4 4 P8207-l 0.542 12349-1 a9. 0 90. 3 54. 0 [8M L816 m P8200-I 0.706 In additiomthree discs of annealed strip from each steel in P8202-l 1.322 the first series of heats, wlth diameters of 3, 3% and 3" 1nches 2 1 54 1174 were drawn into flanged, cylindrical, flat-bottom cups of P8 18 0.179 1.250-inch internal diameter. The average cup depth at frac- 3-5 ture for the three discs of each steel, given in table II], was .944 3,

used as another measure of forming ductility. Likewise, the ul- P8 197-1 0, 0

TABLE V-A Calculated Instability Functions (IF) for the Second Series of Steels Heat No. Calculated IF P8063-IA .l.l4 T555 l-l L69 T55S3-l 2.29 R9593-2 2.62 P8058-B 2.91 R9588-l 2.97 -"P806l-IA TABLE v- Calculated Instability Functions (IF) for the Third Series of Steels As discussed above, the principal feature of our invention is the critical relation between IF and forming ductility. It is observed that for both annealed and cold-rolled strip, the elongation decreases as the IF increases from zero. (A similar relation exists between cup depth and IF.) The graph in the drawing indicates that stable austenitic stainless steels have less ductility than slightly metastable austenitic stainless steels.

Also as shown in the drawing, optimum elongation is achieved only in austenitic stainless steels with an IF inside a rather narrow range. In particular, the greatest elongations are achieved in steels w i t h IF values between zero and2.9. In such steels, the strain-induced transformation of austenite to martensite, with its attendant strengthening of the structure,

achieved. Accordingly, elongations that approach 75 percent can be achieved in such steels. Such great elongations minimize the probability of fracture during the forming of intricate shafi v v It may also be seen from the figure that the elongation of annealed strip drops precipitously to a plateau level of .55 per-' cent when the IF falls below zero. This plateau level, however, is maintained over a broad range of negative IF values. In sta ble austenitic stainless steels, virtually no martensite is formed during straining. Accordingly, these steels do not strengthen at a rate rapid enough to sustain their load-bearing capacity to exceptionally high strains. Thus, the forming operations that can be sustained by stable austenitic stainless steels before fracture are less severe than those tolerated by the metastable steels.

. It may also be seen from the drawing that elongation drops continuously as the IF increases from zero. As the instability of the steel increases, the rate of strain-induced transformation increases. Consequently, the austenite remaining in the structure is quickly depleted. Thereafter, further strengthening of the structure by transformation is very limited. Thus, in moderately metastable steels, the required increase in loadbearing capacity with increasing strain does not extend to exceptionally high strains, and severe forming operations cannot be tolerated.

The practical upper limit of the IF is 2.9. The moderately metastable austenitic stainless steels with-IF values greater than 2.9 generally exhibit no advantage in elongation over the stable austenitic stainlesssteels with negative IF values. The lower limit of the IF is zero.

Calculations of IF values and ductility of hypothetical steels show that both elongation and cup'depth increase as the carbon and/or nitrogen content increases, or as the IF decreases towardzero. Accordingly, the greatest forming ductility is achieved just before the IF reaches zero. However, in such steels, appreciably better ductility is obtained at the higher nitrogen levels than at the higher carbon levels. For example, the two hypothetical steels composed in part of 0.14 percent carbon with 0.06 percent nitrogen, and 0.12 percent carbon with 0.09 percent nitrogen, both exhibit IF values of 0.77. The latter composition, however, offers 43 percent improvement in elongation with an 0.03l-inch improvement in cup depth. This improvement occurs because, for identical [F values, 0.0149 percent nitrogenis the equivalent of 0.01 percent carbon. But 0.0l49percent nitrogen is more effective than 0.0l percent carbon in improving elongation and cup depth.

Although there is advantage to achieving a low IF value at the higher nitrogen contents, there is a practical maximum; limit to the nitrogen content. Increases in nitrogen are more effective than equal increases in carbon in raising the yield strength. Therefore, to avoid yield strengths greater than those normally exhibited by metastable austenitic stainless steels, about 33 to 50 k.s.i., maximum limits on carbon and nitrogen should preferably be about 0.12 and 0.09 percent, respectively. Minimum limits for these elements are then controlled only by the requirement that the IF be within the desired limits. Carbon and nitrogen contents of 0.10 and 0.07 percent," respectively, meet this condition.

Increased amounts of chromium slightly increase the elongation of annealed strip. However, formability reaches a maximum at 17.2 percent chromium. The decreasing tensile strength is offset by an increasing drawing stress as the chromium content increases. The properties of cold rolled steel are not highly sensitive to chromium content. It has been found that optimum formability is achieved at about 17.2 percent chromium, but that formability is quite satisfactory if the chromium content decreases to 16.6 percent or increases to 17.8 percent.

Manganese and nickel are interchangeable in the proportion that 0.4 percent nickel has about the same effect on the IF as 1.0 percent manganese. Thus. a steel that contains 3.0 percent manganese and 6.2 percent nickel should exhibit properties similar to one that contains 1.0 percent manganese and 7.0 percent nickel. The latter proportioning of these two elements is preferable, however, because high manganeselow nickel alloys have poorer hot workability than their low manganese-hi nickel counterparts.

The su trtutron of manganese for nickel reduces forming stresses in both annealed and cold-rolled strip. Also, this substitution improves the ductility of annealed strip; however, it reduces the ductility of cold-rolled strip.

The ductility of annealed strip increases, the tensile strength of both annealed and cold-rolled strip decreases, and the drawing stress of annealed strip decreases as the manganese content increases. The ductility of cold-rolled strip decreases with increasing manganese content. Very similar effects are observed when nickel content is increased. Accordingly, ductility in cold-rolled strip is sacrificed when either manganese or nickel content is increased. Accordingly, ductility in coldrolled strip is sacrificed when either manganese or nickel content is increased. However, the interstitial elements have only desirable effects on properties and the levels of the interstitial elements are kept high. The levels of manganese and nickel, 1.0 and 7.0 percent respectively, are only high enough to provide low positive values of IF. To minimize the risk of exceeding the limits on the IF, or of upsetting previously discussed interaction-dependent results, the ranges of these elements should preferably be narrow; 0.9 to 1.0 percent manganese; 6.75 to 7.25 percent nickel.

We claim:

1. A method for producing for applications involving severe forming, a metastable austenitic stainless steel sheet product, within the range, consisting essentially of 0.07 to 0.18 percent C, 0.9 to 6.2 percent Mn, 4.1 to 7.7 percent Ni, 14.1 to 17.9

percent Cr and 0.01 to 0.14 percent N, balance iron, which comprises,

a. combining the aforesaid elements in proportions in accord with the equation 37.19351.248(% C)l .0l74(% Mn *2.5884(% Ni) 0.4677(% Cr)3.396(% N)=0 to 2.9 to provide a steel product,

b. rolling said steel product to produce a sheet which exhibits a superior combination of low ultimate strength and high ductility as evidenced by an elongation (in 2 inches of 0.035-inch-thick strip) greater than 55 percent, when annealed for 60 seconds at 2,000 F.

2. The method of claim 1, in which said range consists essentially of 0.1 to 0.12 percent C, to 0.9 to 1.10 percent Mn, 6.75 to 7.25 percent Ni, 16.6 to 17.8 percent Cr, and 0.07 to 0.09 percent N.

3. A metastable austenitic stainless steel consisting essentially of 0.1 to 0.12 percent carbon, 0.9 to 1.10 percent manganese, 6.75 to 7.25 percent nickel, 16.6 to 17.8 percent chromium and 0.07 to 0.09 percent nitrogen, the balance essentially iron; said steel having an Instability Function (1F) of 0.0 to 2.9 as determined by the following equation:

-51.248(% C) l.0174(% Mn) -2.5884(% Ni) 0.46770(% Cr) 34.396(% N).

4. A metastable austenitic stainless steel according to claim 3 having nominally 1.0 percent manganese, 7.0 percent nickel and 17.2 percent chromium.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, ,599 ,320 Dated August 17, 1971 Kenneth G. Brickner et a1. Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, Column 4 Table II-B, P8185-l third line third column "18 .56" should read 185.6 Column 5, line 73, should read 1/4 Column 6, Table III, "P8202-2" should read P8202-l Table IV, "P81854-l" should read P8l85-l Column 9, lines 19, 20, 21 cancel "Accordingly, ductility in cold-rolled strip is sacrificed when either manganese or nickel content is increased.". Column 10, line 7, "3.396" should read Signed and sealed this 25th day of July 1972.

(SEAL) Attest:

ROBERT GOTTSCHALK Commissioner of Patents EDWARD M.FLETCHER,JR. Attesting Officer Table II-A, P8l85-1, "40.4" should read 4O .0

ORM PO-1050 (10-69) USCOMM-DC DOING-P69 u,s. GOVERNMENT PRINYING ornc: ma 0-3564 

2. The method of claim 1, in which said range consists essentially of 0.1 to 0.12 percent C, to 0.9 to 1.10 percent Mn, 6.75 to 7.25 percent Ni, 16.6 to 17.8 percent Cr, and 0.07 to 0.09 percent N.
 3. A metastable austenitic stainless steel consisting essentially of 0.1 to 0.12 percent carbon, 0.9 to 1.10 percent manganese, 6.75 to 7.25 percent nickel, 16.6 to 17.8 percent chromium and 0.07 to 0.09 percent nitrogen, the balance essentially iron; said steel having an Instability Function (IF) of 0.0 to 2.9 as determined by the following equation: IF +37.193 -51.248(% C) -1.0174(% Mn) -2.5884(% Ni) -0.46770(% Cr) -34.396(% N).
 4. A metastable austenitic stainless steel according to claim 3 having nominally 1.0 percent manganese, 7.0 percent nickel and 17.2 percent chromium. 